Molecular sieves such as zeolites have been used extensively to catalyze a number of chemical reactions in refinery and petrochemical reactions, and catalysis, adsorption, separation, and chromatography. For example, with respect to zeolites, both synthetic and natural zeolites and their use in promoting certain reactions, including conversion of methanol to olefins (MTO reactions) and the selective catalytic reduction (SCR) of nitrogen oxides with a reductant such as ammonia, urea or a hydrocarbon in the presence of oxygen, are well known in the art. Zeolites are crystalline materials having rather uniform pore sizes which, depending upon the type of zeolite and the type and amount of cations included in the zeolite lattice, range from about 3 to 10 Angstroms in diameter. Zeolites having 8-ring pore openings and double-six ring secondary building units, particularly those having cage-like structures have recently found interest in use as SCR catalysts. A specific type of zeolite having these properties is chabazite (CHA), which is a small pore zeolite with 8 member-ring pore openings (˜3.8 Angstroms) accessible through its 3-dimensional porosity. A cage like structure results from the connection of double six-ring building units by 4 rings.
Catalysts employed in the SCR process ideally should be able to retain good catalytic activity over the wide range of temperature conditions of use, for example, 200° C. to 600° C. or higher, under hydrothermal conditions. Hydrothermal conditions are often encountered in practice, such as during the regeneration of a soot filter, a component of the exhaust gas treatment system used for the removal of particles.
Selective Catalytic Reduction, using ammonia or ammonia precursor as reducing agent is believed to be the most viable technique for the removal of nitrogen oxides from the exhaust of diesel vehicles. In typical exhaust, the nitrogen oxides are mainly composed of NO (>90%), so the SCR catalyst favors the conversion of NO and NH3 into nitrogen and water. Two major challenges in developing catalysts for the automotive application of the ammonia SCR process are to provide a wide operating window for SCR activity, including low temperatures of from 200° C. and higher and improvement of the catalyst's hydrothermal stability for temperatures above 500° C. As used herein hydrothermal stability refers to retention of a material's capability to catalyze the SCR of NOx, with a preference for the retention to be at least 85% of the material's NOx conversion ability prior to hydrothermal aging.
Metal-promoted zeolite catalysts including, among others, iron-promoted and copper-promoted zeolite catalysts, where, for instance, the metal is introduced via ion-exchange, for the selective catalytic reduction of nitrogen oxides with ammonia are known. Iron-promoted zeolite beta has been an effective catalyst for the selective reduction of nitrogen oxides with ammonia. Unfortunately, it has been found that under harsh hydrothermal conditions, such as reduction of NOx from gas exhaust at temperatures exceeding 500° C., the activity of many metal-promoted zeolites, such as Cu and Fe versions of ZSM-5 and Beta, begins to decline. This decline in activity is believed to be due to destabilization of the zeolite such as by dealumination and consequent loss of metal-containing catalytic sites within the zeolite.
To maintain the overall activity of NOx reduction, increased levels of the washcoat loading of the iron-promoted zeolite catalyst must be provided. As the levels of the zeolite catalyst are increased to provide adequate NOx removal, there is an obvious reduction in the cost efficiency of the process for NOx removal as the costs of the catalyst rise.
Due to the considerations discussed above, there is a desire to prepare materials which offer improved low temperature SCR activity and/or improved hydrothermal durability over existing zeolitic materials, for example, catalyst materials which are stable at temperatures up to at least about 650° C. and higher, for example in the range of about 700° C. to about 800° C. and up to about 900° C. Moreover, since diesel engines operate under transient conditions, there is a desire to provide materials that exhibit high performance over a wide temperature range, from as low as 200° C. up to about 600° C. Thus, while existing technologies provide high temperature performance, there is a need for materials can offer low temperature performance in predominantly NO feeds combined with hydrothermal stability. Low temperature performance is important for cold start and low engine load conditions.
While silicoaluminophosphate (SAPO) materials have attracted some interest for SCR of NOx, one limitation of these materials is that these materials can become unstable when exposed to humid or moist environments at temperatures below 100° C. Thus, it would be desirable to provide a catalyst material that can provide excellent low temperature SCR of NOx to meet current governmental (for example, Euro 6) NOx regulations. Additionally, it would be desirable to provide an SCR catalyst that is not prone to extensive deactivation under moist conditions at low temperature.